Abstract
The lateral nucleus of the trapezoid body (LNTB) is a prominent nucleus in the superior olivary complex in mammals including humans. Its physiology in vivo is poorly understood due to a paucity of recordings. It is thought to provide a glycinergic projection to the medial superior olive (MSO) with an important role in binaural processing and sound localization. We combined in vivo patch clamp recordings with labeling of individual neurons in the Mongolian gerbil. Labeling of the recorded neurons allowed us to relate physiological properties to anatomy at the light and electron microscopic level. We identified a population of quite dorsally located neurons with surprisingly large dendritic trees on which most of the synaptic input impinges. In most neurons, one or more of these dendrites run through and are then medial to the MSO. These neurons were often binaural and could even show sensitivity to interaural time differences (ITDs) of stimulus fine structure or envelope. Moreover, a subpopulation showed enhanced phase-locking to tones delivered in the tuning curve tail. We propose that these neurons constitute the gerbil main LNTB (mLNTB). In contrast, a smaller sample of neurons was identified that was located more ventrally and that we designate to be in posteroventral LNTB (pvLNTB). These cells receive large somatic excitatory terminals from globular bushy cells. We also identified previously undescribed synaptic inputs from the lateral superior olive. pvLNTB neurons are usually monaural, display a primary-like-with-notch response to ipsilateral short tones at CF and can phase-lock to low frequency tones. We conclude that mLNTB contains a population of neurons with extended dendritic trees where most of the synaptic input is found, that can show enhanced phase-locking and sensitivity to ITD. pvLNTB cells, presumed to provide glycinergic input to the MSO, get large somatic globular bushy synaptic inputs and are typically monaural with short tone responses similar to their primary input from the cochlear nucleus.
Highlights
The superior olivary complex (SOC) is a collection of brainstem nuclei concerned primarily with auditory processing
None of the posteroventral LNTB (pvLNTB) neurons of our limited example was tuned to the very low frequencies at which interaural time difference (ITD)-sensitivity is most marked, we studied these neurons with tones below characteristic frequency (CF)
Because main LNTB (mLNTB) neurons are often binaural and can display phase-locking to monaural stimuli, we examined whether they have ITD tuning
Summary
The SOC is a collection of brainstem nuclei concerned primarily with auditory processing. The medial and lateral superior olive (MSO and LSO), are crucial components of sound localization circuitry. They are surrounded by a series of nuclei designated periolivary (Warr, 1966; Guinan et al, 1972b; Tsuchitani, 1977; Spirou and Berrebi, 1996; Kulesza, 2007, 2008). MSO neurons are sensitive to time differences in the arrival of sound at the two ears (ITDs) by a process akin to coincidence detection on inputs from each side (Yin and Chan, 1990; Franken et al, 2015). The function of inhibition on MSO cells has been the subject of intense debate (Brand et al, 2002; Joris and Yin, 2007; Pecka et al, 2008; Roberts et al, 2013; Myoga et al, 2014; Franken et al, 2015)
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